WJEC Biology for A2: Student Bk

Going further In auto-immune conditions, the adaptive immune system does not recognise the body’s tissues as self, and treats them as non-self, so they are attacked by antibodies. Going further Many organisms have an innate immune system: recognition of ‘non-self’ has been seen in all animal phyla and in plants. Phagocytic cells, e.g. macrophages and granulocytes, may have their evolutionary origin in organisms resembling Amoeba . The innate immune system The innate immune system is a group of natural barriers that resist infection in several ways: ▪ The skin covers the external surface of the body, except at openings such as the mouth and the eyes. – Keratin in epidermal cells makes the skin waterproof. – Collagen in the connective tissue of the dermis, maintained by vitamin C, makes the skin tough. ▪ The skin flora, or microbiota, comprises bacteria and fungi, which outcompete pathogenic strains. Unlike the pathogens, they are not easily washed off and so regular washing remains an important way of resisting infection. ▪ Inhaled air is a source of micro-organisms and their spores. Mucus traps them and the cilia of the epidermal cells lining the respiratory passages bring them up to the opening of the oesophagus, from where they are swallowed. ▪ When the skin barrier is breached: – If capillaries are broken, blood clots prevent the entry of microbes. – Inflammation sets in. Increased blood flow towards the site of infection brings large numbers of phagocytic cells. The broken capillaries heal and the raised temperature is unfavourable to microbes. ▪ If microbes enter the blood stream, phagocytic cells including macrophages and neutrophils, engulf and digest them. ▪ Tears, mucus and saliva contain lysozyme, an enzyme that hydrolyses peptidoglycan molecules in bacterial cell walls, and kills them. ▪ Stomach acid kills many of the microbes that are ingested in food and drink. The adaptive immune system Important components of the adaptive immune system ‘Adaptive’ means that the body produces a specific response to each antigen, so the body is adapting. Lymphocytes provide this response. They are derived from stem cells in the bone marrow and their precise role depends on their subsequent location. Option A: Immunology and disease 271